Error field penetration and plasma rotation in H mode and control of ELMfree regimes in COMPASS-D

Error fields are observed to form locked modes at abnormally high densities in ohmic ELM-free H modes in COMPASS-D. Recent experiments are described where the penetration is correlated with measured toroidal rotation changes intrinsic to the H mode in COMPASS-D. Once penetrated, the error field triggers ELMs (often followed by a return to L mode). This often leads to disruptions, but can be a cyclic process. The experiments on COMPASS-D show that an error field can be used to produce ELMs in a controlled way.